Polyurethanes are a well-known type of adhesive. They contain precursor materials that cure in place to form an adhesive layer. Polyurethane adhesives come in one-part and two-part types. One-part types generally exhibit a moisture cure or a heat-activated cure. Two-part types consist of a resin component that includes one or more polyisocyanate compounds, and a curative component that includes one or more polyols. When the two components are mixed, the polyisocyanates and polyols react to form a cured polyurethane adhesive. A polyurethane adhesive can be formulated to cure at room temperature or upon exposure to certain conditions, an example of which is an elevated temperature. As the adhesive cures, it can form a strong adhesive bond to many types of substrates.
Two part curable compositions are used in a variety of applications such as adhesives, coatings, foams and the like. Two part compositions are used where rapid cure is required for the application, especially where the two parts are not shelf stable when in contact with one another. Shelf stable means that the composition does not cure in storage. It is desirable that the adhesive composition exhibit a suitable open time and cure rapidly. The “open time” refers to the amount of time after the two components are mixed that the adhesive remains flowable and capable of bonding to a substrate.
One way of obtaining both a long open time and a fast cure is by formulating the adhesive to have a heat-activated cure. Such an adhesive cures slowly at ambient temperature, thereby allowing the adhesive to be applied and the substrates positioned while the adhesive remains flowable. The resulting assembly is then heated to an elevated temperature at which rapid curing takes place.
Heating using conventional curing ovens tends to be slow and expensive, and may not be well-suited for large or complex assemblies. To lower costs and speed the cure, infrared heating methods have been developed. These methods permit the substrate/adhesive assembly to be brought more rapidly to the curing temperature than with convection ovens.
Infrared heating methods can be used to partially cure the adhesive to develop enough initial bond strength to allow the adhered assembly to be handled in downstream manufacturing operations, while allowing full cure to develop later. This approach can speed production processes, as it is not necessary to wait for the adhesive to fully cure before subsequent manufacturing steps can take place.
Infrared heating has a further advantage in that it can be targeted, so only specific locations of the assembly are heated. This allows for spot-curing, i.e., curing only predetermined portions of the adhesive. Strong localized adhesive bonds form where the localized heat is applied. Even though much of the adhesive remains uncured or only partially cured, these localized bonds provide enough strength to the assembly that it can be manipulated. This process saves time, because localized heating can be accomplished more rapidly than curing the entire assembly, and saves energy as only a portion of the assembly needs to be heated. The subsequent full cure can take place at ambient temperature, or if an elevated temperature cure is needed, this full curing step sometimes can be combined with another manufacturing step, such as a paint curing step, to save costs and speed production rates.
Two part fast curing two part adhesives are disclosed in WO2014/040909 and US2015/0203728. Such two part adhesive systems provide limited flexibility. Process flexibility may be defined as long open time that is the time between application of the adhesive to a first substrate and joining of a second substrate to the first substrate using the adhesive. Further long mixer stand-alone times, the time the mixed two part adhesive can be kept in the mixer unit (static or dynamic) between two applications without gelling, are required to reduce the flushing intervals and therefore reduce waste. Fast cure, as evidenced by fast strength build up, once the open time window closes is desired to provide handling strength as soon as possible after final assembly of the components.
Thus, what is needed is a two part adhesive that provides long open times, sufficient cure strength after one hour to allow handling of bonded parts and low reactivity loss after storage for at least one month. What are needed are bonding methods using two part adhesives that allow reasonable time to contact and locate substrates to one another with the adhesive disposed between the substrates, wherein the substrates can be handled after about one hour and the adhesive retains significant reactivity for at least a month such that final cure of the adhesive bond can be achieved after at least a month.